Conventionally, an ordinary nuclear medicine diagnostic apparatus includes a gamma-ray detector in a circular or polygonal ring form for detecting gamma rays, and an image processor for producing an RI distribution image based on emission data obtained from the gamma-ray detector. The gamma-ray detector includes a scintillator with numerous scintillator chips for emitting light upon incidence of gamma rays, and a photomultiplier tube for converting the light emissions of the scintillator into electric signals.
A patient is placed in the hollow portion of the gamma-ray detector, and the patient is injected with a radioactive drug labeled with a positron-emitting radioisotope. The positron-emitting radioisotope distributed in the body releases two gamma rays in 180-degree opposite directions. The gamma-ray detector detects the gamma rays released from the patient. Then, the image processor collects, as emission data, events of a pair of gamma rays counted as coincidence, and produces a two-dimensional or three-dimensional RI distribution image based on this emission data. The RI distribution image produced is suitable mainly for diagnosing presence or absence, position, malignancy and so on of a tumor.
Conventionally, there is also a nuclear medicine diagnostic apparatus used mainly for radiographing the breasts (hereinafter called the nuclear medicine diagnostic apparatus for mammography). In the case of breast cancer also, it is effective to find a smaller tumor for the sake of early detection. Therefore, the size of the scintillator chips is reduced, and the gamma-ray detector is constructed to be set close to the patient (see Patent Document 1, for example).
[Patent Document 1]
    Japanese Unexamined Patent Publication No. 2003-325499